An optical recording medium for writing and reading information uses a radiation beam having a predetermined wavelength and a predetermined numerical aperture. The medium includes a recording layer and the recording layer is changed from a first to a second state upon irradiation by a radiation beam. The recorded information is represented by written marks in the second state within a region in the first state. The marks are arranged in one or more tracks including a guide groove having a width and a depth. A first optical phase difference exists between the reflection from a region on the tracks in the first state and from a region on the tracks in the second state. The first optical phase difference enhances an optical phase difference between a region in between the tracks in the first state and the region on the tracks in the first state.
Information may be stored in such a recording medium by a scanning device having an optical head. The head focuses a radiation beam onto the information layer in the medium and follows an unwritten track using tracking information derived from the groove in the track. When the medium is disc-shaped, the grooves are circular or spiral and the tracking information includes a radial tracking error signal. When a relatively high power radiation beam is modulated by a signal representing the information to be written, the information is written in the tracks as optically detectable marks. During reading, the radiation beam has a relatively low power, which, on reflection from the information layer, is modulated by the marks. The tracking information may be derived during reading from the grooves or from the written information.
An optical recording medium in Japanese patent application JP-A 5174380 includes a stack of optical thin layers in which the recording layer is embedded. The thickness of a transparent layer of the stack adjacent the recording layer is tuned such that the first optical phase difference between unwritten and written regions of a track increases a second optical phase difference between a region in between the tracks in the first state and a region on the tracks also in the first state. This relation between the phase differences increases the information signal derived from the scanned marks.
Generally, for single layer media, information for controlling tracking is obtained by the push-pull method or the phase detection method. The push-pull method for deriving tracking information is known from U.S. Pat. No. 4,057,833. The phase-detection method for deriving tracking information is known from U.S. Pat. No. 4,785,441.
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